By way of example, electronic ballasts known from the prior art use a boost converter as a voltage converter for the apparatus for power factor correction (PFC). In this case, reasonably constant intermediate-circuit voltages can be produced with the aid of a slow I regulator. In this case, the time constant of the I regulator must be long in order to obtain PFC characteristics of the voltage converter, that is to say to allow a sinusoidal current to be drawn. In the event of sudden load changes, for example on those which occur on ignition and the subsequent operation of the discharge lamp, the intermediate-circuit voltage must first of all drop considerably before the I regulator can react. The slow I characteristic of the regulator furthermore produces a further time delay before the closed-loop control system starts to operate. During the process, the intermediate-circuit voltage falls to an undesirably great extent. In the case of load circuits designed on the basis of limit values, this then leads to undesirable operating points. This can result in the discharge lamp not igniting, or in the electronic ballast switching off.
In order to prevent this, appropriate reserves are incorporated in the load circuit in the prior art. The load circuit in the prior art must therefore be designed such that it can produce an adequate power, for example for ignition of the lamp, even at relatively low intermediate-circuit voltages. For this purpose, by way of example, the capacitor at the output of the voltage converter is designed to be large, as is the lamp inductor. These measures allow more reactive current to be produced, to still allow ignition of the discharge lamp even when the intermediate-circuit voltage is low. The high reactive current which flows in this case results in undesirably high losses. The efficiency of an electronic ballast such as this is undesirably low, because of this.
WO 02/098187 A1 and EP 0 596 740 A1 disclose circuit arrangements for closed-loop control of the lamp power, for example in order to take account of ageing of the discharge lamp.